The X-TERRA is a very flexible detector in that it will allow the user to determine how much "programming" you want to do. If you want to hunt in the factory presets, they'll work fine. After you "get the feel" for it, you can tweak your X-TERRA to make it as "refined" as you need, for any site you decide you want to detect. If your soil is iron rich, my suggestion would be to buy the Gold Pack instead of the standard 705 package. I believe they are the same price and I know they are the same detector. But the Gold Package includes a 6 X 10" Elliptical DD coil at 7.5 kHz as the stock coil. The stock coil in the "regular" package is the 9-inch concentric at 7.5 kHz. Although any of the DD coils are great at target separation, my preference for the real nasty areas is the 6-inch DD at 18.75 kHz. Currently there are 9 "authorized" coils available for the X-TERRA. 8 from Minelab and one from Coiltek.
I don't see learning the X-TERRA as being a curve. It is just a matter of understanding how a notch discrimination system works and setting up the detector for your site. With the 705, you simply turn it on and it can chose the best noise cancel channel and even set the ground balance automatically. Set up the Threshold for your hearing, the volume for how loud you want to signal to be, the Sensitivity as high as you need (without causing falsing), and you're ready to hunt. When you detect a target, the X-TERRA assigns a value to each target. That value is associated with one of the 28 notch segments. You determine beforehand which notch segments you want to accept (beep) and which notch segments you want to ignore (no beep). They don't have to be "in a row". The great thing about notch discrimination is that it allows the user to accept (for example) nickels, copper and silver, and reject pull tabs. On variable discrimination such as on the Vaquero, when you set the discrimination control, it rejects everything below that level. With notches, you can pick and chose which target values you want to hear.
If you take a look near the top of these posts, you'll see one that is FAQFAQ. It has posts that address many of the most frequently asked questions. Just above that, at the top of list of posts, you'll see one about my eBook, Understanding your X-TERRA. This book is available on Minelab's website, FREE of charge. All you have to do is log in download it. Take a look at it and see if it addresses some of the issues or questions you have about the X-TERRA. If it doesn't, make a post! There are a LOT of very knowledgeable X-TERRA users on this forum. HH Randy